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The destruction caused by the Indian Ocean tsunami in December 2004 and Hurricane Katrina in August 2005 stunned observers around the world. These extreme events served as reminders that tsunamis and coastal storms are two of the most dangerous hazards to coastal population centers and economic infrastructures. U.S. Geological Survey (USGS) scientists have been studying geologic impacts from extreme wave events and have recently published a paper that will help researchers distinguish ancient tsunami deposits from large-storm deposits in the geologic recorda capability that is vital for assessing the threat that an area faces from these two hazards.
Both tsunamis and large storms cause death and damage along low-lying coastal areas, and both occur with regularity, although tsunamis are much less frequent than coastal storms. Because of their infrequency, tsunamis are poorly documented in historical records for many of the areas where they pose a threat. In these areas, interpreting the geologic record may be the only way to discover the history of past tsunamis and the likely hazard from future tsunamis. Both tsunamis and large storms, particularly hurricanes, are capable of inundating coastal areas and depositing sandy sediment over broad areas landward of the beach. Correctly identifying a sandy bed in the geologic record as either a tsunami or a storm deposit is critical for determining the frequency of each hazard.
The new paper, "Physical Criteria for Distinguishing Sandy Tsunami and Storm Deposits Using Modern Examples," was released August 2007 in a special issue of the journal Sedimentary Geology titled "Sedimentary Features of Tsunami DepositsTheir Origin, Recognition, and Discrimination" (v. 200, no. 3-4). The authorsgeologist Robert Morton (St. Petersburg, Florida) and oceanographers Guy Gelfenbaum (Menlo Park, California) and Bruce Jaffe (Santa Cruz, California)were motivated to conduct this study by comments from tsunami scientists who speculated that tsunami and storm deposits are too similar to be able to distinguish the origin of deposition. After discovering common research interests during a USGS Coastal and Marine Geology Program science-planning workshop in 2002 (see Sound Waves article, "Coastal and Marine Geology Program Planning Meeting, Palo Alto, CA"), Morton and Gelfenbaum initiated discussions with coastal scientists Jaffe and Bruce Richmond (USGS) and James Goff (National Institute of Water and Atmosphere [NIWA] Science, New Zealand) that eventually led to this comparative analysis of modern events. (See related article in Sound Waves, October 2002, "Group Aims to Distinguish Tsunami Deposits from Large-Storm Deposits in the Geologic Record.")
Morton, Gelfenbaum, and Jaffe began their work by examining modern deposits whose originstsunami or large stormare known. The new paper compares deposits from recent tsunamis in Papua New Guinea (1998) and Perú (2001) with deposits from Hurricane Carla in the Gulf of Mexico (1961) and Hurricane Isabel in the western Atlantic Ocean (2003). The authors conclude that certain physical characteristics may, indeed, be useful for distinguishing the two types of deposit. These characteristics include sediment composition, texture, and grading (how grain size changes from bottom to top); types and organization of sediment layers; deposit thickness and geometry; and whether the deposit drapes the preexisting landscape or levels it by filling in low places.
Tsunami deposits are generally less than 25 cm thick, extend hundreds of meters inland from the beach, and have an overall tendency to drape the preexisting landscape. They commonly consist of a single, homogeneous bed that grades from coarser grained at the bottom to finer grained at the top, or a bed with only a few thin layers. Mud clasts or thin layers of mud within the deposit are strong evidence of tsunami origin. Twig orientation or other indicators of return (seaward) flow during deposition of the sediment are also diagnostic of tsunami deposits. Tsunami deposits thicken and then thin landward, with a maximum deposit thickness typically more than 50 m inland from the beach because a zone of erosion commonly is present near the beach.
Storm deposits, in contrast, generally are more than 30 cm thick and will not advance beyond the low places they are able to fill in the preexisting topography. They typically consist of multiple laminasetssets of extremely thin (less than 1 cm thick) layers called laminae. Features that favor storm deposits are the types of stratification associated with the transport of sediment by rolling and bouncing along the bottom (foresets, climbing ripples, backsets), and numerous thin (millimeters to a few centimeters) laminasets of alternating coarse and fine grain size indicative of high-frequency waves. Abundant shell fragments organized in laminae also favor a storm origin. Storm deposits contain no internal mud layers and rarely contain pieces of mud. Maximum deposit thickness is near the shore, and landward thinning of the deposit is commonly abrupt. Storm deposits fill in topographic lows, and the upper surface is relatively uniform in elevation alongshore.
These distinguishing characteristics are relatively easy to spot in recent deposits exposed over large areas but are trickier to identify in limited exposures of ancient, buried deposits. Tsunami and large-storm deposits have many similarities, and it is unlikely that unequivocal diagnostic attributes will be preserved at any single observation sitefor example, in a trench dug to reveal buried layers. Multiple sample sites and a quasi-three-dimensional reconstruction of the sedimentary deposit in question would likely be required to adequately evaluate the origin of an ancient deposit. At many locations, the most reliable means of differentiating tsunami and storm deposits may be the context in which the deposit occurs. A sandy deposit associated, for example, with features produced by a large earthquakeincluding liquefaction structures and evidence of subsidence, such as buried soils and drowned forestswould likely have a tsunami origin.
The recent paper emphasizes the use of physical attributes to differentiate between tsunami- and storm-emplaced sand deposits. Other studies have examined the use of microfossil assemblages, pollen, and geochemical signatures as evidence for marine inundation and onshore sediment transport caused by tsunamis and storms. The authors note, "Perhaps combining complementary physical, paleontological, and chemical data will someday allow unequivocal differentiation of tsunami and storm deposits."
The full citation for the new paper is: Morton, R.A., Gelfenbaum, Guy, and Jaffe, B.E., 2007, Physical criteria for distinguishing sandy tsunami and storm deposits using modern examples, in Tappin, D.R., ed., Sedimentary features of tsunami depositstheir origin, recognition, and discrimination: Sedimentary Geology, v. 200, no. 3-4 (special issue), p. 184-207 [URL http://www.sciencedirect.com/science/journal/00370738].
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